Yevamot 46B

בטבל ולא מל ⁠— כולי עלמא לא פליגי דמהני, כי פליגי במל ולא טבל. רבי אליעזר יליף מאבות, ורבי יהושע: באבות נמי טבילה הוה.

With regard to one who immersed but was not circumcised, everyone, i.e., both R' Yehoshua and R' Eliezer, agrees that the halakha is derived from the foremothers that immersion alone is effective. Where they disagree is with regard to one who was circumcised but had not immersed; R' Eliezer derives that it is effective from the forefathers, and R' Yehoshua disagrees because he maintains that in the conversion of the forefathers there was also an immersion.

מנא ליה? אילימא מדכתיב: ״לך אל העם וקדשתם היום ומחר וכבסו שמלתם״, ומה במקום שאין טעון כבוס ⁠— טעון טבילה, מקום שטעון כבוס ⁠— אינו דין שטעון טבילה.

The Talmud asks: From where did he derive this? If we say that he derived it from the fact that it is written that in preparation for the revelation at Sinai, God commanded Moses: “Go to the people and sanctify them today and tomorrow, and let them wash their garments” (Exodus 19:10), as R' Yehoshua understands that the washing mentioned in this verse is the ritual immersion of clothes, this leads to the following a fortiori inference: Just as in a case where one became impure through contact with some source of impurity, washing, i.e., immersion, of clothes is not required but immersion of one’s body is required, then in a case where washing of clothes is required, as in the preparation for the revelation at Sinai, isn’t it logical that immersion of one’s body should also be required?

ודלמא, נקיות בעלמא?!

The Talmud rejects the proof: But perhaps when the verse states that they had to wash their clothes, it was merely for cleanliness and not for the sake of ritual purity. If so, no a fortiori inference can be drawn from it to the case of immersion for ritual purity.